Spinach (Spinacia oleracea) has become an important vegetable crop in many parts of the world, with the top spinach producing county being China (>80% of global production), followed by USA, Japan and various European countries. Globally about 1 million ha of spinach are grown in Asia and about 35,000 ha in each of the EU, USA and Japan (see Correll et al., 2011, Eur J Plant Pathol 129: 193-205). Part of the increase in spinach demand is likely due to an increased health-consciousness of consumers and the beneficial properties of spinach. Spinach leaves are rich in beta-carotene, lutein, folic acid, vitamin C, calcium, iron and antioxidants. Especially the demand for fresh spinach has significantly increased over the last years.
Due to this increase in production over the last 10-15 years, incidence and severity of one of the most damaging pathogens of spinach, downy mildew of spinach, caused by races of the oomycete Peronospora farinosa f.sp. spinaciae (Pfs; synonym P. effusa) has increased concomitantly. Before 1990 only three races of Pfs were known, however between 1990 and 2010 ten new races were identified. The emergence of new races of Pfs makes this pathogen a major threat for spinach production globally and identifying new sources of resistance is therefore necessary.
Historically, Pfs race 1 (Pfs: 01 or Pfs 1) was first reported in 1824 and resistance to race 1 was identified later in two Iranian accessions (PI140467 and PI140464), and incorporated into commercial hybrid varieties, such as Califlay (Smith and Zahara, California Agriculture, July 1956). In 1958, race 2 appeared and a few years later a single dominant gene imparting resistance against both race 1 and race 2 was identified (Smith et al. 1961 and 1962). In 1976, race 3 appeared and again several years later resistance against race 3 was found. Race 4 was only identified in 1990, and Brandenberger et al. (1992) identified accessions CGN09546, of which 60% of individual plants were resistant, and SP1 82/87, of which 80% of individual plants were resistant. The rapid emergence of new races hereafter, lead to the identification of new resistance genes and their incorporation into commercial varieties, as indicated in Table 1 below (− means resistant reaction; + means susceptible reaction; (−) means reduced level of infection often referred to as field resistance, i.e. sparse sporulation on the tips of the cotyledons; +/− means undecided). These varieties are also used as host differentials for determining the race of isolates of Pfs (see document of the International Seed Federation, August 2013; world wide web at worldseed.org/isf/differential_hosts.html).
TABLE 1PfsPfsPfsPfsPfsPfsPfsPfsPfsPfsPfsPfsPfsPfsVariety1234567891011121314Viroflay++++++++++++++Resistoflay−−++++++++++++Califlay−+−+−++−−+−−+−Clermont−−−−++++++++++Campania−−−−−+−+++−++/−+Boeing−−−−−−−+−+−+−+(=Avengerin USA)Lion−−−−−−−−−+−−−−Lazio−−−−−−−−−−++++Whale−−−(−)−(−)(−)−−+−−+(−)
Commercial spinach varieties are mostly hybrids, produced by crossing a male and a female inbred line, although also some open pollinated varieties exist. The male and female parent line generally each carry a different resistance gene. For example, the hybrid variety Andromeda (Nunhems; see patent application US2012/0222147) is resistant against Pfs 1-12 and Pfs14. Resistance against Pfs 1, 3, 5, 8, 9, 11, 12 and 14 is conferred by a resistance gene from one parent, while resistance against Pfs 1-10 is conferred by a resistance gene from the other parent.
WO2013/064436 describes a dominant resistance gene, called R6, which confers resistance against Pfs1-6, 9, 11-14 (see Table 1 on page 19 of WO2013/064436; in 2011 the type strain UA4410 has been designated Pfs14 by the International Working Group on Peronospora farinosa, IWGP).
To achieve resistance against all known Pfs races (Pfs1-14) using the known resistance genes, one would for example need to combine the resistance genes of Lazio and Lion. To date, no single resistance gene is known, which confers resistance against all known Pfs races, or especially against Pfs races 7-14.